1. Field of the Invention
The present invention relates to a color image reading apparatus. Specifically, the invention relates to a color image reading apparatus suitably applicable to a color scanner, a color facsimile machine, and the like, which uses a color separator comprising a blazed diffraction grating and detection means comprising three line sensors disposed on a surface of a single substrate and which may read color image information on a plane of an original at a high precision without color blur.
2. Related Background Art
There have been proposed various reading apparatuses in which color image information on a plane of an original is formed through an optical system on line sensors such as CCD and the color image information is digitally read using output signals of the line sensors.
For example, Japanese Patent Publication No. 62-43594 describes a color image detection apparatus in which color image information is color-separated by using a blazed diffraction grating as an optical element for color separation and in which the thus-separated color image information is guided to three monolithic line sensors to detect the color image information. U.S. Pat. Nos. 4,277,138 and Des. 2,645,075 also disclose a color image reading apparatus using such a blazed diffraction grating as color separation means.
The method for color separation using the blazed diffraction grating, in which an incident optical flux is separated into a plurality of color lights as diffraction light of certain order, is in general characterized by easy color separation in a simple arrangement.
When the incident optical flux is diffracted and color-separated by the blazed diffraction grating, spectral characteristics and angles of emission of color light beams obtained depend upon conditions such as an incident angle of the optical flux into the blazed diffraction grating, a grating pitch of the diffraction grating, and a diffraction order. Thus, the incident optical flux must be incident at a certain angle onto the blazed diffraction grating for example, in order to guide the plural color lights color-separated by the blazed diffraction grating to the respective line sensors in a satisfactory precision.
However, in case that a color image is formed by a projection optical system on the line sensors while being color-separated into color images based on a plurality of color lights through the blazed diffraction grating, optical fluxes from so-called off-axial images neighboring the object color image on the plane of the original also enter the blazed diffraction grating through the projection optical system at different angles.
Those optical fluxes from the off-axial images are color-separated by the blazed diffraction grating and then enter the line sensors to become color blur of unnecessary color lights (for example of .+-. first order diffraction light). The color blur results in decrease in reading precision of the color image, which has been problematic in the conventional apparatuses.